1. Field of the Invention
The present invention relates to an organic electroluminescent device (EL device) which can emit light by converting electric energy into optical energy.
2. Description of the Related Art
Since an organic electroluminescent (EL) device can emit high-brightness light with a low voltage, the organic electroluminescent device has attracted attentions as a promising display device. External quantum efficiency is an important characteristic of the organic electroluminescent device. The external quantum efficiency is calculated as “external quantum efficiency Φ=the number of photons emitted from a device/the number of electrons injected into the device,” and it can be said that it is more advantageous in view of power consumption as the external quantum efficiency becomes greater.
The external quantum efficiency of the organic electroluminescent device is defined by “external quantum efficiency Φ=internal quantum efficiency×light extraction efficiency.” In an organic electroluminescent device using fluorescent light emission from an organic compound, since the limit value of the internal quantum efficiency is about 25% and the light extraction efficiency is about 20%, the limit value of the external quantum efficiency is about 5%.
In order to further improve characteristics of light emitting devices, there has been suggested a green light emitting device using light emission from tris-ortho-metalated complex of iridium (III) with 2-phenylpyridine (Ir(ppy)3) (for example, see U.S. Patent Application Publication No. 2002/0034656 A1). The phosphorescent light emitting device disclosed in U.S. Patent Application Publication No. 2002/0034656 A1 has green and red light emission efficiency improved much greater than that of a conventional singlet light emitting device, but improvement in durability thereof has been required.